By use of a closed circulatory system, animal bodies use many internal organs and vessels to transport fluids from one bodily location to another. Components of the circulatory system include the heart, blood vessels, and blood. The heart has valves (semilunar valves and atrioventricular valves) that regulate the flow of blood in the atria and the ventricles. Three examples of blood vessels are arteries, veins, and capillaries. Whereas arteries transport blood to organs throughout the body (i.e., away from the heart), veins carry blood back to the heart. Structurally, capillaries have an inner endothelium surrounded by a membrane, while arterial and venal walls have three layers: connective tissue forms the outer layer, while smooth muscle having elastic fibers forms the middle layer, and there is an innermost endothelium layer. Mammalian veins, such as human veins for example, have naturally occurring valves positioned along the length of the vessel.
Mammalian valves, such as human venous valves for example, act as one-way check valves that open to permit the flow of fluid in a first direction (e.g., muscles contract, squeeze the veins, and the valves—flaps of tissue—keep blood moving toward the heart), and quickly close upon a change in pressure, such as a transition from systole to diastole or when muscles relax or stop contraction, to prevent fluid flow in a reverse direction, i.e., retrograde flow.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present and can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include the use of compression stockings that are placed around the leg of a patient in an effort to force the vessel walls radially inward to restore valve function. Surgical techniques are also employed in which valves can be bypassed or replaced with autologous sections of veins with competent valves.
Minimally invasive techniques and instruments for placement of intraluminal medical devices have developed over recent years. A wide variety of treatment devices that utilize minimally invasive technology has been developed and includes stents, stent grafts, occlusion devices, infusion catheters and the like. Minimally invasive intravascular devices have especially become popular with the introduction of coronary stents to the U.S. market in the early 1990s. Coronary and peripheral stents have been proven to provide a superior means of maintaining vessel patency, and have become widely accepted in the medical community. Furthermore, the use of stents has been extended to treat aneurysms and to provide occlusion devices, among other uses.
Therefore, it is desirable to have prosthetic valve devices for implantation in a body vessel as taught herein, and methods of making such devices.